This invention relates to measurement of a fluid characteristic and, more particularly, to a method and apparatus that compensates for pressure changes in the fluid being measured.
A known technique for measuring fluid characteristics such as fluid density and fluid flow rate is to pass the fluid through a tube and to set up vibrations in the tube. The resonant frequency of the tube depends upon the inherent characteristics of the tube and the fluid passing through the tube. For example, as the density of the fluid increases the effective mass of the tube also increases and the resonant frequency of the tube decreases. The stresses on the tube also affect the resonant frequency. These stresses are caused by various factors, the hydro-static pressure within or on the measurement tube, its temperature, and the whole densimeter""s mounting hardware. For example, as the fluid pressure increases, the spring constant of the tube increases and the resonant frequency of the tube increases. In order to make the resonant frequency of the tube representative of the fluid density independent of pressure, the fluid pressure must be measured.
Typically, the fluid pressure is measured by inserting a pressure transducer inside the tube in contact with the fluid. As a result, the pressure transducer is exposed to the fluid, which can be intolerable if the fluid has destructive characteristics, i.e., if the fluid is corrosive or abrasive. Furthermore, in order to gain access to the pressure transducer for inspection, repair or replacement, the fluid system must be dismantled.
According to the invention, a tube is installed in a fluid system to ascertain changes in a characteristic of the fluid contained in the tube, such as resonant frequency. The fluid pressure in the tube is sensed by one or more strain gauges mounted on the exterior wall of the tube. The strain gauges measure the strain in the tube, which is proportional to the pressure exerted by the fluid on the tube. The measured strain is used to compensate for pressure induced changes in the tube""s characteristics.
In one embodiment which senses fluid density, a tube through which fluid flows is installed in a fluid system. A piezoelectric or magnetic driver is mounted on the exterior of the tube. A piezoelectric or magnetic sensor is also mounted on the exterior of the tube. A feedback loop from the sensor to the driver is adapted to cause vibrations in the tube at its resonant frequency or a harmonic thereof. One or more strain gauges mounted on the exterior wall of the tube senses the strain exerted on the exterior wall by the pressure of the fluid flowing through the tube. A microprocessor determines the density of the fluid flowing through the tube responsive to the tube""s motion, temperature sensor, and the one or more strain gauges.
In its preferred embodiment, the one or more strain gauges comprise a bridge circuit. Strain gauges comprise two arms of the bridge circuit and temperature dependent resistors comprise the other two aims of the bridge circuit. The output from the one or more strain gauges is used to correct the frequency reading given by the piezoelectric sensor.